Am 24.06.2017 um 23:50 schrieb John Brooks:
On Sat, Jun 24, 2017 at 08:20:22PM +0200, Christian König wrote:
Am 24.06.2017 um 01:16 schrieb John Brooks:
On Fri, Jun 23, 2017 at 05:02:58PM -0400, Felix Kuehling wrote:
Hi John,
I haven't read your patches. Just a question based on the cover letter.
I understand that visible VRAM is the biggest pain point. But could the
same reasoning make sense for invisible VRAM? That is, doing all the
migrations to VRAM in a workqueue?
Regards,
Felix
I don't see why not. In theory, all non-essential buffer moves could be done
this way, and it would be relatively trivial to extend it to that.
But I wanted to limit the scope of my changes, at least for this series.
Testing takes a long time and I wanted to focus those testing efforts as much
as possible, produce something well-tested (I hope), and get feedback on this
limited application of the concept before expanding its reach.
Yeah, sorry I have to say that but the whole approach is utterly nonsense.
What happens is that the delayed BO can only be moved AFTER the command
submission which wants it to be in VRAM.
So you use the BO in a CS and *then* move it to where the CS wants it to be,
no matter if the BO is then needed there or not.
Regards,
Christian.
I'm aware of the effect it has. The BO won't be in VRAM for the current command
submission, but it'll be there for a future one. If a BO is used at a given
time, then it's likely it'll be used again soon.
Exactly that's the problem here. Keep in mind that BOs can only move in
between command submissions.
So instead of moving the BO on the first command submission which needs
it you most likely move it directly after it.
It is actually quite unlikely that a BO which was swapped out will be
used for multiple command submissions in a row (well it was swapped out
because it was unused for a while).
In which case you'll come out
ahead on latency even if the GPU has to read it from GTT a few times. In any
case, it's never going to hurt as much as full-stop waiting for a worst-case BO
move that needs a lot of evictions.
Feel free to correct my understanding; you'd certainly know any of this better
than I do. But my tests indicate that immediate forced moves during CS cause
stalls, and the average framerate with delayed moves is the almost (~2%) the
same as with immediate ones, which is about 9% higher than with no forced moves
during CS at all.
Your understanding is incorrect.
The stalls are not caused by the BO moves itself, but rather by the
ping/pong they result in and the intermediate waits this causes.
See memory bandwidth is usually not the problem, but swapping a BO out
needs to be reflected in all VMs the BO is using. This adds extra
dependencies in between command submissions, so instead of rendering and
copying at the same time you get everything serialized.
I'm pretty sure that this is actually the root cause of the issues you
run into here.
Beside limiting those ping/pongs to something reasonable per second
another possible solution would be to make more room for CPU accessible
VRAM BOs. I will take a look into this next week, Felix is kicking me
for this for a while anyway.
Regards,
Christian.
DiRT Rally average framerates:
With the whole patch set (n=3):
89.56
Without it (drm-next-4.13 5ac55629d6b3fcde69f46aa772c6e83be0bdcbbf)
(n=3):
91.16 (+stalls)
Patches 1 and 3 only, and with GTT set as the only busy placement for
CPU_ACCESS_REQUIRED BOs in amdgpu_cs_bo_validate (n=3):
82.15
John
John
On 17-06-23 01:39 PM, John Brooks wrote:
This patch series is intended to improve performance when limited CPU-visible
VRAM is under pressure.
Moving BOs into visible VRAM is essentially a housekeeping task. It's faster to
access them in VRAM than GTT, but it isn't a hard requirement for them to be in
VRAM. As such, it is unnecessary to spend valuable time blocking on this in the
page fault handler or during command submission. Doing so translates directly
into a longer frame time (ergo stalls and stuttering).
The problem worsens when attempting to move BOs into visible VRAM when it is
full. This takes much longer than a simple move because other BOs have to be
evicted, which involves finding and then moving potentially hundreds of other
BOs, which is very time consuming. In the case of limited visible VRAM, it's
important to do this sometime to keep the contents of visible VRAM fresh, but
it does not need to be a blocking operation. If visible VRAM is full, the BO
can be read from GTT in the meantime and the BO can be moved to VRAM later.
Thus, I have made it so that neither the command submission code nor page fault
handler spends time evicting BOs from visible VRAM, and instead this is
deferred to a workqueue function that's queued when CS requests BOs flagged
AMDGPU_GEM_CREATE_CPU_ACCESS_REQUIRED.
Speaking of CPU_ACCESS_REQUIRED, I've changed the handling of that flag so that
the kernel driver can clear it later even if it was set by userspace. This is
because the userspace graphics library can't know whether the application
really needs it to be CPU_ACCESS_REQUIRED forever. The kernel driver can't know
either, but it does know when page faults occur, and if a BO doesn't appear to
have any page faults when it's moved somewhere inaccessible, the flag can be
removed and it doesn't have to take up space in CPU-visible memory anymore.
This change was based on IRC discussions with Michel.
Patch 7 fixes a problem with BO moverate throttling that causes visible VRAM
moves to not be throttled if total VRAM isn't full enough.
I've also added a vis_vramlimit module parameter for debugging purposes. It's
similar to the vramlimit parameter except it limits only visible VRAM.
I have tested this patch set with the two games I know to be affected by
visible VRAM pressure: DiRT Rally and Dying Light. It practically eliminates
eviction-related stuttering in DiRT Rally as well as very low performance if
visible VRAM is limited to 64MB. It also fixes severely low framerates that
occurred in some areas of Dying Light. All my testing was done with an R9 290
with 4GB of visible VRAM with an Intel i7 4790.
--
John Brooks (Frogging101)
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